Halogenated polymeric materials such as poly(tetrafluoroethylene) (PTFE) and poly(trifluoromonochloroethylene) are attractive candidates for advanced electronic packaging applications because of their very low dielectric constants, excellent chemical stability, low solvent/moisture absorption and excellent thermal stability. In addition, certain composite halogenated polymer compositions employing composite PTFE materials using fillers such as glass or ceramic micro-particles have improved dimensional stability and a low thermal expansion coefficient (CTE).
For instance, a glass/ceramic filled poly(tetrafluoroethylene) available under the trade designation RO2800.TM./has a CTE (x-y) value of 16 ppm/.degree.C. which is closely matched to the 16.9 ppm/.degree.C. value for copper metal. This enhances the thermal cycle reliability of the RO2800.TM./CU interface. The use of these materials in high performance packaging or multilevel structures would provide reduced signal delay and rise times, reduced cross talk at a given impedance, and increased circuit density. A significant enhancement in the reliability of the packaging structures would be gained because of the low water uptake by the polymer. This would tend to eliminate corrosion problems, hygroscopic expansion, and improved metal to dielectric adhesion reliability.
However, because of their relative chemical inertness and hydrophobic nature, these halogenated polymeric materials are difficult to process into electronic packaging structures. The lack of effective processing techniques has inhibited the exploitation of these materials by the electronics industry. The low surface energy of these materials gives the inability to adhere to other surfaces and must be effectively overcome to yield desirable metal adhesion for practical electronic packaging applications.
In the manufacture of printed circuit cards and boards, a dielectric sheet material is employed as the substrate. A conductive circuit pattern is provided on one or both of the major surfaces of the substrate. Since the dielectric substrate is non-conductive, in order to plate on the substrate, it must be seeded or catalyzed prior to the deposition of metal thereon.
Among the more widely employed procedures for catalyzing a substrate is the use of a stannous chloride sensitizing solution and a palladium chloride activator to form a layer of metallic palladium particles thereon. For instance, one method of catalyzing a dielectric substrate is exemplified by U.S. Pat. No. 3,011,920 which includes sensitizing a substrate by first treating it with a solution of a colloidal metal, activating the treatment with a selective solvent to remove unreactive regions from the colloids on the sensitized dielectric substrate, and then electrolessly depositing a metal coating on the sensitized substrate, for example, with copper from a solution of a copper salt and a reducing agent.
Also, as suggested, for example, in U.S. Pat. No. 3,009,608, a dielectric substrate can be pretreated by depositing a thin film of a "conductivator" type of metal particle, such as palladium metal, from a semicolloidal solution onto the dielectric substrate to provide a conducting base which permits electroplating with conductive metal on the conductivated base.
In addition, there have been various suggestions of treating substrates with certain materials in order to enhance the attachment to the substrate of a non-noble metal catalyst. For instance, U.S. Pat. No. 4,301,190 suggests a pre-wet treatment of a substrate with an "absorption modifier" to enhance the attachment to the substrate of a non-noble metal catalyst. Certain surfactants, hydrous oxide sols and certain complexing agents are suggested as "absorption modifiers".
However, the methods of catalyzing, or seeding, various organic polymer substrates have not been entirely satisfactory and improvement in the degree of adhesion of the final metal layer to the substrate has been less than desired.
This is especially true for polyhaloalkylene containing substrates such as poly(tetrafluoroethylene) (PTFE), and in fact, the lack of effective processing techniques has inhibited the effective use of these polymeric materials by the electronics industry. The hydrophobic nature and low energy of the surfaces of poly(haloalkylene) polymers render such quite difficult to metallize or bond to metal layers normally resulting in poor adhesion of metal layers to the surface.
It has been suggested, for example, in British Patent 793,731 and further suggested, for example, by A. A. Benderly, J. Appl. Polymer Science, 6, 221 (1962), to treat PTFE with very strong reducing species such as elemental alkali metals such as sodium in liquid ammonia or sodium-naphthalene in tetrahydrofuran solutions in order to increase the surface energy of the surface and render such "wettable" to improve adhesive bonding to metals, plastics, wood and glass.
The use of alkali metal in liquid ammonia solutions has been used to treat PTFE films for improving the adhesion for pressure-sensitive tape applications such as suggested by Fields in U.S. Pat. No. 2,946,710. Methods of "activating" perfluorocarbon polymer surfaces for improved bonding towards organic adhesive coatings by treating with alkali metals, magnesium and zinc at elevated temperatures in amine solvents or ammonia has been reported by Purvis et al. in U.S. Pat. No. 2,789,063. Rappaport in U.S. Pat. No. 2,809,130 suggests methods for improving bonding between fluorinated resins and other materials by treating surfaces with an alkali metal polyaryl hydrocarbon-solvent solution. However, none of these prior art suggestions involves using subsequent in situ reduction of a chemical modification of the fluoropolymer surface for use as a catalyst for subsequent deposition of seed metal.
Dousek et al., Electrochimica Acta, 18, 1 (1975), discussed the use of alkali metals and alkali amalgams to treat PTFE which leads to a hydrophilic "carbonaceous" surface. Because alkali metals react explosively upon contact with water liberating hydrogen gas these systems are extremely dangerous. Commercially available sodium naphthalide solutions such as TetraEtch.RTM. (W. L. Gore and Associates) are ethylene glycol dimethyl ether solutions such as monoglyme have very low flash points (e.g. 34.degree. F.) and react violently with water. The highly reactive nature of alkali metal-liquid ammonia and sodium naphthalide solutions along with the large capital cost required for safety and dangers associated with handling the raw reagents and waste effluent make these treatments prohibitive under industrial safety regulations.
Accordingly, the safety controls and concerns that are necessary for such a process along with the high equipment costs involved render such techniques highly unattractive from a manufacturing viewpoint.
Alternative vacuum or plasma treatment processes have the disadvantages of requiring high cost vacuum equipment and have a low throughput capability. Furthermore, such treatments are limited to altering only the outermost few atomic layers of the surface and the resulting surface modification are unstable and undergo additional changes within hours.
As described in, for instance, U.S. Pat. No. 3,689,991 and Tummala, et al. "Microelectronics Packaging Handbook", pp. 409-435, Van Nostrand Reinhold, flexible polymeric films can be used as carriers in the packaging of semiconductor chips such as in the so-called TAB (Tape Automated Bonding) procedure. To date, the primary polymeric material employed for such has been polyimide.
One procedure used for employing polyimide as the dielectric and/or circuit carrier for flexible circuits involves spray coating or roller coating polyamic acid onto a sheet of metal (such as stainless steel or aluminum). The film is then cured or imidized, resulting in a film which is fully or substantially fully cured. The metal which the polyimide is on can be imaged, removed, or maintained. On top of the polyimide, three layers of metal are deposited such as by either evaporation or sputtering. The conductors are chromium or nickel, followed by a layer of copper, followed by a layer of chromium or nickel. By means of photolithographic operations, this metal is imaged into circuits. Depending on the use of the circuit, the cured polyimide may or may not be imaged, either before or after the formation of the circuit.
Flexible circuits have also been fabricated using free-standing polymeric films such as polyimides onto which metal layers are vacuum deposited, laminated, or glued. The metal circuit pattern is defined by using a photoresist pattern to either act as a plating mask or act as a mask for subtractive etching of the metal layer. Through-holes in the polymer film can be made by drilling, punching, or etching.
In a number of these situations, it is necessary to form vias in the polymeric layer to allow for electrical connections to be made between the different layers of metallurgy. In order that the interconnection be as accurate as possible, it is necessary that the polymeric films resist distortion of the desired pattern and withstand attack from other wet processing chemicals.
For instance, in the formation of multi-layer substrates for mounting chips it is necessary to electrically contact some of the conductors in the upper or second layer of metallization to some of the conductors on the lower or first layer of metallization. In order to do so, the polymeric layer must be selectively etched to form the desired vias therein to allow for metal connection between the upper and lower levels of metallization and connection to a chip and/or board.
In TAB structures certain regions (windows) must be etched in the polymer layer in order to expose metal bonding leads such as the inner and outer leads to allow both chip attachment to the TAB package and TAB package attachment to a circuit card. Caustic solutions are commonly used to fabricate such windows in TAB structures containing polyimide as the dielectric.
Conventional electronic packages have conductors comprised of defined metal regions and might have surface mounted capacitors and resistors attached. The direct conversion of certain regions of the dielectric to a conductive material would allow fabrication of planar electronic components (i.e., resistors) to be directly fabricated on the dielectric surface without requiring extra components to be attached.
Wet etching of various poly(halogenated) olefinic polymers such as poly(tetrafluoroethylene) on a commercial basis has been carried out employing alkali metals, such as sodium naphthalide or liquid ammonia solutions. However, such processes suffer from the disadvantages discussed above.